Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

One embodiment takes the form of a system for locating wireless
transmitters employing an Orthogonal Frequency Division Multiplexing
(OFDM) digital modulation scheme, which comprises transmitting signal
components over narrowband frequency channels spanning a wideband
channel. The system includes a first receiving system configured to
receive a fraction of the signal components transmitted by a first
wireless transmitter to be located in a fraction of the narrowband
frequency channels, and to process the fraction of the signal components
to derive location related measurements. The system further includes at
least a second receiving system configured to receive the fraction of the
signal components transmitted by the first wireless transmitter, and to
process this fraction of the signal components to derive location related
measurements.

Claims:

1. A location measuring unit (LMU) for use in a system for locating
wireless transmitters employing an Orthogonal Frequency Division
Multiplexing (OFDM) digital modulation scheme, wherein said OFDM scheme
comprises transmitting signal components over a plurality of narrowband
frequency sub-channels spanning a wideband channel having a bandwidth of
approximately 20 MHz or greater, wherein a high-rate bit stream to be
transmitted by a wireless transmitter using said OFDM scheme is split
into multiple low-rate bit streams and transmitted in parallel over
multiple sub-channels, wherein each low-rate bit stream is transmitted
over a narrowband sub-channel by modulating a sub-carrier using a digital
modulation scheme, comprising: a receiving system configured to receive a
fraction of the signal components transmitted by a first wireless
transmitter to be located in a fraction of the narrowband sub-channels;
and a processor configured to process said fraction of the signal
components to derive location related measurements; wherein the LMU is
configured to cause the receiving system to receive signal components in
a plurality of selected narrowband sub-channels, wherein the signal
components in the selected narrowband sub-channels are received in
parallel.

2. An LMU as recited in claim 1, wherein said fraction of the narrowband
frequency sub-channels includes at least one pilot channel in which said
first wireless transmitter transmits energy, and wherein the LMU is
configured to use signal components in said pilot channel to aid in
signal acquisition and demodulation.

3. An LMU as recited in claim 2, wherein the selected sub-channels are
determined based upon interference levels.

4. An LMU as recited in claim 1, wherein the receiving system comprises a
first frequency conversion circuit configured to convert a first received
RF signal component to a first digital baseband OFDM signal; a Fast
Fourier Transform (FFT) circuit configured to perform an FFT of the first
digital baseband OFDM signal provided by the first frequency conversion
circuit; a demodulation circuit configured to produce coded bits based on
the output of the FFT circuit; and a first reconstruction circuit
configured to reconstruct the first digital baseband OFDM signal based on
the coded bits produced by the demodulation circuit.

5. An LMU as recited in claim 1, wherein said location related
measurements comprise at least one of the following: time difference of
arrival (TDOA) measurements, time of arrival (TOA) measurements, angle of
arrival (AOA) measurements, round trip time measurements, and received
power measurements.

6. An LMU as recited in claim 1, wherein said fraction of the predefined
narrowband frequency channels excludes guard channels in which said first
wireless transmitter transmits minimal energy.

7. An LMU as recited in claim 1, further comprising a radio frequency
(RF) filter and an intermediate frequency (IF) filter, wherein the
receiving system is configured to receive signal components within a
bandwidth compatible with said RF and IF filters.

8. An LMU as recited in claim 1, further comprising an analog to digital
converter (ADC) characterized by a sample rate, wherein the receiving
system is configured to receive signal components within a bandwidth
compatible with said sample rate.

9. An LMU as recited in claim 8, wherein said ADC is further
characterized by a sample rate after decimation, wherein the receiving
system is configured to receive signal components within a bandwidth
compatible with said sample rate after decimation.

10. An LMU as recited in claim 1, further comprising memory for storing
data representing received signal components, wherein said receiving
system is configured to receive signal components within a bandwidth
compatible with said available memory.

11. An LMU as recited in claim 1, further comprising a digital signal
processor (DSP) characterized by DSP processing throughput, wherein said
receiving system is configured to receive signal components within a
bandwidth compatible with said DSP processing throughput.

12. An LMU as recited in claim 1, further comprising means for
communicating said location related measurements to a processing system
for use in computing the location of said first wireless transmitter.

13. An LMU as recited in claim 1, wherein each of the plurality of
narrowband subchannels has a bandwidth that is substantially less than 20
MHz.

14. An LMU as recited in claim 13, wherein each of the plurality of
narrowband subchannels has a bandwidth that is less than approximately 5
MHz.

15. An LMU as recited in claim 13, wherein each of the plurality of
narrowband subchannels has a bandwidth in the range of approximately 3-5
MHz.

16. A method for use in a system for locating wireless transmitters
employing an Orthogonal Frequency Division Multiplexing (OFDM) digital
modulation scheme, wherein said OFDM scheme comprises transmitting signal
components over a plurality of narrowband sub-channels spanning a
wideband channel having a bandwidth of approximately 20 MHz or greater,
wherein a high-rate bit stream to be transmitted by a wireless
transmitter using said OFDM scheme is split into multiple low-rate bit
streams and transmitted in parallel over multiple sub-channels, wherein
each low-rate bit stream is transmitted over a narrowband sub-channel by
modulating a sub-carrier using a digital modulation scheme, comprising:
receiving a fraction of the signal components transmitted by a first
wireless transmitter to be located in a fraction of the narrowband
sub-channels, wherein the receiving includes receiving signal components
in a plurality of selected narrowband sub-channels, wherein the signal
components in the selected narrowband sub-channels are received in
parallel; and processing said fraction of the signal components to derive
location related measurements.

17. A method as recited in claim 16, wherein said fraction of the
narrowband sub-channels includes at least one pilot channel in which said
first wireless transmitter transmits energy.

18. A method as recited in claim 17, further comprising using signal
components in said pilot channel to aid in signal acquisition and
demodulation.

19. A method as recited in claim 16, further comprising converting a
received RF signal component to a digital baseband OFDM signal;
performing a Fast Fourier Transform (FFT) of the digital baseband OFDM
signal; producing coded bits based on the output of the FFT; and
reconstructing the digital baseband OFDM signal based on the coded bits.

20. A method as recited in claim 16, wherein said location related
measurements comprise at least one of the following: time difference of
arrival (TDOA) measurements, time of arrival (TOA) measurements, angle of
arrival (AOA) measurements, round trip time measurements, and received
power measurements.

21. A method as recited in claim 16, wherein said fraction of the
predefined narrowband sub-channels excludes guard channels in which said
first wireless transmitter transmits minimal energy.

22. A method as recited in claim 16, wherein each of the plurality of
narrowband subchannels has a bandwidth that is substantially less than 20
MHz.

23. A method as recited in claim 22, wherein each of the plurality of
narrowband subchannels has a bandwidth that is less than approximately 5
MHz.

24. A method as recited in claim 22, wherein each of the plurality of
narrowband subchannels has a bandwidth in the range of approximately 3-5
MHz.

25. A method as recited in claim 16, further comprising communicating
said location related measurements to a processing system for use in
computing the location of said first wireless transmitter.

Description:

CROSS-REFERENCE

[0001] This application is a continuation of and claims priority to U.S.
application Ser. No. 12/909,732, filed Oct. 21, 2010, currently pending,
which is a continuation of U.S. application Ser. No. 11/609,817, filed
Dec. 12, 2006, now U.S. Pat. No. 7, 844, 280, the contents of both of
which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

[0002] The present invention relates generally to the field of wireless
communications, and more specifically to the location of wireless devices
within the coverage area of a wireless communications network. Wireless
devices, also called mobile stations (MS), include those such as used in
analog or digital cellular systems, personal communications systems
(PCS), enhanced specialized mobile radios (ESMRs), wide-area-networks
(WANs), and other types of wireless communications systems. This field is
now generally known as wireless location, and has application for
Wireless E911, fleet management, RF optimization, security, and other
valuable applications.

BACKGROUND

[0003] A. Wireless Location

[0004] Early work relating to Wireless Location Systems is described in
U.S. Pat. No. 5,327,144, Jul. 5, 1994, "Cellular Telephone Location
System," which discloses a system for locating cellular telephones using
time difference of arrival (TDOA) techniques. This and other exemplary
patents (discussed below) are assigned to TruePosition, Inc., the
assignee of the present invention.

[0005] The '144 patent describes what may be referred to as an
uplink-time-difference-of-arrival (U-TDOA) cellular telephone location
system. The described system may be configured to monitor control channel
transmissions from one or more cellular telephones and to use central or
station-based processing to compute the geographic location(s) of the
phone(s). For example, in station-based processing, which may be employed
for reverse control channel signal detection, cross-correlations are
performed at the cell sites (or signal collection systems) in the
following manner: For each "strong" signal, which may be considered a
reference signal, received on a particular control channel at a
particular first cell site, that strong signal is first applied to a
signal decoder, such as that used by the cellular system itself This
decoder demodulates the cellular signal to produce the original digital
bit stream which had been modulated to produce the cellular signal. This
digital bit stream is then modulated by the cell site system to
reconstruct the original signal waveform as it was first transmitted by
the cellular telephone. This reconstructed signal waveform is
cross-correlated against the received signal at the first cell site. The
cross-correlation produces a peak from which an exact time of arrival can
be calculated from a predetermined point on the peak. The first cell site
system then sends the demodulated digital bit stream and the time of
arrival to the central site over the communications line. The central
site then distributes the demodulated digital bit stream and the exact
time of arrival to other cell sites likely to have also received the
cellular transmission. At each of these other second, third, fourth,
etc., cell sites, the digital bit stream is modulated by the cell site
system to reconstruct the original signal waveform as it was first
transmitted by the cellular telephone. This reconstructed signal waveform
is cross-correlated against the signal received at each cell site during
the same time interval. The cross-correlation may or may not produce a
peak; if a peak is produced, an exact time of arrival (TOA) can be
calculated from a predetermined point on the peak. This TOA is then sent
to the central site, and a delay difference, or TDOA, for a particular
pair of cell sites can be calculated. This method permits the cell site
systems to extract TOA information from an extremely weak signal
reception, where the weak signal may be above or below the noise level.
This method is applied iteratively to sufficient pairs of cell sites for
each strong signal received at each cell site for each sample period. The
results of the delay pairs for each signal are then directed to the
location calculation algorithm.

[0006] TruePosition and others (e.g., KSI, Inc.) have continued to develop
significant enhancements to the original inventive concepts. Some
examples are discussed below.

[0007] U.S. Pat. No. 6,047,192, Apr. 4, 2000, "Robust, Efficient,
Localization System," is another example of a prior art patent describing
a similar process (referred to as "matched-replica processing") for
processing mobile transmitter signals to determine location related
signal parameters, which may be employed to calculate the transmitter
location.

[0008] Another exemplary prior art patent is U.S. Pat. No. 6,091,362, Jul.
18, 2000, "Bandwidth Synthesis for Wireless Location System." This patent
describes a system and process offering improved accuracy of location
information and greater time resolution. In the described system, signals
transmitted by wireless telephones are received at a plurality of signal
collection sites. To improve the accuracy of the location information,
the system synthesizes greater bandwidth, and thus greater time
resolution, than would otherwise be available. The location system can
issue commands to cause a wireless transmitter to be located to change
frequency channels, and a doubly-differenced carrier phase of the
transmitted signal, or the TDOA, is observed at each of many frequencies
spanning a wide bandwidth. The phase-measurement data from these many
frequencies are combined to resolve the inherent integer-wavelength
ambiguity. The invention may be utilized to obtain a bandwidth greater
than the typical bandwidth of the signals to be cross-correlated (in
either the time or frequency domains) in a cellular telephone location
application.

[0009] Another example is U.S. Pat. No. 6,646,604, Nov. 11, 2003,
"Automatic Synchronous Tuning of Narrowband Receivers of a Wireless
Location System for Voice/Traffic Channel Tracking." This patent
describes a transmitter locating method that involves performing location
processing on signals received during an automatic sequential tuning mode
of operation, wherein narrowband receivers are tuned sequentially and in
unison to a plurality of predefined RF channels. Signal transmissions of
interest in these channels are digitally recorded and used in location
processing. The identity of the located transmitter(s) is determined by
matching a location record to data indicating which wireless transmitters
were in use at a time corresponding to the location record, and which
cell sites and RF channels were used by each wireless transmitter.

[0010] An example of a wireless location system (WLS) of the kind
described above is depicted in FIG. 1. As shown, the system includes four
major subsystems: the Signal Collection Systems (SCS's) 10, the TDOA
Location Processors (TLP's) 12, the Application Processors (AP's) 14, and
the Network Operations Console (NOC) 16. Each SCS is responsible for
receiving the RF signals transmitted by the wireless transmitters on both
control channels and voice channels. In general, an SCS (now sometimes
called an LMU, or Location Measuring Unit) is preferably installed at a
wireless carrier's cell site, and therefore operates in parallel to a
base station. Each TLP 12 is responsible for managing a network of SCS's
10 and for providing a centralized pool of digital signal processing
(DSP) resources that can be used in the location calculations. The SCS's
10 and the TLP's 12 operate together to determine the location of the
wireless transmitters. Both the SCS's 10 and TLP's 12 contain a
significant amount of DSP resources, and the software in these systems
can operate dynamically to determine where to perform a particular
processing function based upon tradeoffs in processing time,
communications time, queuing time, and cost. Each TLP 12 exists centrally
primarily to reduce the overall cost of implementing the WLS. In
addition, the WLS may include a plurality of SCS regions each of which
comprises multiple SCS's 10. For example, "SCS Region 1" includes SCS's
10A and 10B that are located at respective cell sites and share antennas
with the base stations at those cell sites. Drop and insert units 11A and
11B are used to interface fractional T1/E1 lines to full T1/E1 lines,
which in turn are coupled to a digital access and control system (DACS)
13A. The DACS 13A and another DACS 13B are used in the manner described
more fully below for communications between the SCS's 10A, 10B, etc., and
multiple TLP's 12A, 12B, etc. As shown, the TLP's are typically
collocated and interconnected via an Ethernet network (backbone) and a
second, redundant Ethernet network. Also coupled to the Ethernet networks
are multiple AP's 14A and 14B, multiple NOC's 16A and 16B, and a terminal
server 15. Routers 19A and 19B are used to couple one WLS to one or more
other Wireless Location System(s).

[0011] B. Evolving Wireless Standards and Air Interface Protocols

[0012] Over the past few years, the cellular industry has increased the
number of air interface protocols available for use by wireless
telephones, increased the number of frequency bands in which wireless or
mobile telephones may operate, and expanded the number of terms that
refer or relate to mobile telephones to include "personal communications
services," "wireless," and others. The air interface protocols now used
in the wireless industry include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS,
ESMR, GPRS, EDGE, UMTS WCDMA, and others. UMTS is a wideband CDMA air
interface protocol defined by ETSI 3GPP. This protocol is similar to the
CDMA protocols in EIA/TIA IS-95, or CDMA 2000, but does not require
synchronization of the base stations, and also provides a high level of
interoperability with GSM network infrastructure.

[0013] Orthogonal Frequency Division Multiplexing (OFDM) is a multiplexing
technique in which a given subscriber may be assigned many frequency
channels over which it will simultaneously transmit. The multiplexing
scheme provides high bandwidth efficiency and broadband wireless
communication in a high multi-path environment. WiFi as defined in IEEE
802.11 and WiMax as defined in IEEE 802.16 utilize OFDM. It is expected
that IEEE 802.20 (when re-ratified) will utilize OFDM.

[0014] Uplink TDOA location of fourth generation (4G) broadband OFDM
signals with bandwidths that can exceed 20 MHz requires expensive
receiver and signal processing hardware. The SCSs (or LMUs) may be
required to receive, sample, store and process these broadband signals,
making the hardware significantly more expensive than what is required
for third generation (3G) signals, such as UMTS or CDMA 2000 WCDMA
signals occupying a bandwidth of 3-5Mhz. As described in greater detail
below, a goal of the present invention is to provide a way to accomplish
U-TDOA location on the broadband 4G waveforms by collecting and
processing only a portion of the transmitted signal, reducing the
required bandwidth, memory, and digital signal processing required in the
SCS/LMU, while still achieving high accuracy.

SUMMARY

[0015] The following summary is intended to explain several aspects of the
illustrative embodiments described in greater detail below. This summary
is not intended to cover all inventive aspects of the disclosed subject
matter, nor is it intended to limit the scope of protection of the claims
set forth below.

[0016] One illustrative embodiment of the present invention takes the form
of a system for locating wireless transmitters employing an Orthogonal
Frequency Division Multiplexing (OFDM) digital modulation scheme. The
OFDM scheme comprises transmitting signal components over a plurality of
predefined narrowband frequency channels spanning a predefined wideband
channel. The system includes a first receiving system configured to
receive a fraction of the signal components transmitted by a first
wireless transmitter to be located in a fraction of the predefined
narrowband frequency channels, and to process the fraction of the signal
components to derive location related measurements. The system further
includes at least a second receiving system configured to receive the
said fraction of the signal components transmitted by the first wireless
transmitter, and to process this said fraction of the signal components
to derive location related measurements. The system also includes a
processing system configured to use location related measurements from
the first and second receiving systems to compute the location of the
wireless transmitter.

[0017] Other aspects of the embodiments disclosed herein are described
below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The foregoing summary as well as the following detailed description
are better understood when read in conjunction with the appended
drawings. For the purpose of illustrating the invention, there is shown
in the drawings exemplary constructions of the invention; however, the
invention is not limited to the specific methods and instrumentalities
disclosed. In the drawings:

[0019] FIG. 1 schematically depicts a Wireless Location System.

[0020] FIGS. 2A and 2B are block diagrams of the signal processing in OFDM
transmitters and receivers, respectively.

[0021]FIG. 3 illustrates a reduced spectrum processed by a SCS or LMU as
compared with the entire spectrum transmitted.

[0023]FIG. 5 is a block diagram of a modified signal processing chain
employed to support a reduced signal bandwidth.

[0024] FIG. 6 is a block diagram of a reconstruction process for the
reduced signal.

[0025] FIG. 7 is a flowchart of a station-based location process for the
reduced signal.

[0026] FIG. 8A depicts an ideal cross-correlation function showing peaks
due to two signal components, a direct path component and a delayed
component due to a multi-path reflection.

[0027] FIG. 8B depicts an ideal cross-correlation function (solid line)
showing peaks due to two signal components and a band-limited
cross-correlation function showing the smearing of those peaks that make
them indistinguishable.

[0028]FIG. 8c depicts an ideal cross-correlation function (solid line)
showing peaks due to two signal components and a band-limited
cross-correlation function, with 4× the bandwidth of the function
shown in FIG. 8B, still showing some smearing, but the increased
bandwidth makes two individual peaks distinguishable.

[0029] FIG. 9 illustrates the full bandwidth of an OFDM waveform with
small slices processed at any one time by an SCS/LMU, with multiple time
intervals used to cover most or all of the OFDM waveform bandwidth.

[0030]FIG. 10 schematically depicts a Wireless Location System for
locating OFDM transmitters in accordance with an illustrative embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0031] We will now describe illustrative or presently preferred
embodiments of the present invention. First, we provide an overview and
then a more detailed description.

[0032] A. Overview

[0033] The present invention may be embodied in various forms, e.g., as a
system, method, or computer readable medium bearing executable
instructions for carrying out the inventive process. For example, a
system in accordance with the present invention may be implemented as a
system for locating wireless transmitters employing an Orthogonal
Frequency Division Multiplexing (OFDM) digital modulation scheme. The
illustrative system is shown schematically in FIG. 10. The OFDM scheme
comprises transmitting signal components over a plurality of narrowband
frequency channels spanning a wideband channel. The system includes first
and second receiving systems (elements 100 and 101 in FIG. 10), which may
take the form of an SCS or LMU co-located at a base transceiver station
of a wireless communications system, although this is by no means
required. The receiving systems are each configured to receive a fraction
of the signal components transmitted by a wireless transmitter to be
located (element 120 in FIG. 10) in a fraction of the narrowband
frequency channels, and to process the signal components to derive
location related measurements. These measurements are then provided to a
processing system (element 110) configured to use the location related
measurements to compute the location of the wireless transmitter. The
processing system may take the form of a TLP of the kind referred to
above, although this is not required.

[0034] The location related measurements derived by the receiving systems
may comprise measurements of time difference of arrival (TDOA), time of
arrival (TOA), angle of arrival (AOA), round trip time, power, or another
form of measurement that may be used to calculate the location of the
wireless transmitter.

[0035] The fraction of the narrowband frequency channels received by the
receiving systems may include at least one pilot channel in which the
wireless transmitter transmits energy, and the receiving systems may be
configured to use signal components in the pilot channel to aid in signal
acquisition and demodulation. Moreover, the fraction of the narrowband
frequency channels may exclude guard channels in which the wireless
transmitter transmits minimal energy.

[0036] The receiving systems may each include a radio frequency (RF)
filter, and they are preferably each configured to receive signal
components within a bandwidth compatible with the RF filter. The
receiving systems may also include an intermediate frequency (IF) filter,
and are preferably configured to receive signal components within a
bandwidth compatible with the IF filter. In addition, the receiving
systems may each include an analog to digital converter (ADC)
characterized by a sample rate, and are preferably configured to receive
signal components within a bandwidth compatible with the sample rate. The
ADCs may be characterized by a sample rate after decimation, and the
receiving systems may be configured to receive signal components within a
bandwidth compatible with the sample rate after decimation. The receiving
systems may also include available memory for storing data representing
received signal components, and may be configured to receive signal
components within a bandwidth compatible with the available memory. The
receiving systems may also include digital signal processors (DSPs)
characterized by DSP processing throughput, and may be configured to
receive signal components within a bandwidth compatible with the DSP
processing throughput.

[0037] The receiving systems may be configured to receive signal
components within a bandwidth compatible with a current load on the
receiving system. For example, the amount of DSP processing available
within the receiving system (e.g., SCS) at any point in time may be a
function of the location processing load on the system. If the load
happens to be lower, and adequate DSP processing resources are available,
then a wider portion of the transmitted bandwidth could be processed.
However, if the load on the receiving system is high, a smaller portion
of transmitted bandwidth would be processed to reduce the processing load
on the DSP resources.

[0038] The receiving systems may also be configured to tune to a plurality
of channels to receive signals from a plurality of wireless transmitters
to be located. In addition, the receiving systems may be configured to
tune to a plurality of selected channels, wherein the selected channels
are determined based upon interference levels. For example, higher
interference could reduce the ability for receiving systems (e.g., LMUs)
to detect signals, and could reduce the accuracy of computed locations.
In general, it is better to select the portion of transmitted spectrum
that is least used by other transmitters. The level of interference over
different sections of the transmitted signal can be determined by making
power measurements at the receiving system, and/or by using the knowledge
of the channels used by other transmitters in the network. The wireless
network itself should have knowledge of the spectrum utilization.

[0039] The selected channels may be determined based upon various factors,
including but not limited to measurements of received signals and
spectrum usage.

[0040] A bandwidth synthesis process may also be advantageously employed
in connection with the present invention.

[0041] Moreover, use of the present invention may also involve use of a
sequential or random pattern of re-tuning a frequency agile receiver to
cover most or all of the OFDM waveform spectrum.

[0042] In addition, a station-based or central processing method may be
advantageously used in practicing the invention.

[0043] B. Location of Broadband OFDM Transmitters with TDOA, using only a
Portion of the Transmitted Spectrum

[0044] Broadband wireless communication infrastructure is being deployed
and used on a large scale basis. WiFi capable devices, as defined in IEEE
802.11G, are capable communicating at rate of 54 mbps using a signal
bandwidth on the order of 20 MHz. WiMAX capable devices as defined in
IEEE 802.16 will be capable of communicating at rate of 75 mbps, with
signal bandwidth on the order of 20 MHz. This broadband capability will
allow higher throughput applications to be used by wireless devices.
Robust location techniques such as U-TDOA are needed for these mobile
devices for emergency and other location based services.

[0045] Orthogonal frequency-division multiplexing (OFDM), also sometimes
called discrete multitone modulation (DMT), is based upon the principle
of frequency-division multiplexing (FDM), but is often used as a digital
modulation scheme. The bit stream that is to be transmitted is split into
several parallel bit streams, typically dozens to thousands, and the
available frequency spectrum is divided into several sub-channels, and
each low-rate bit stream is transmitted over one sub-channel by
modulating a sub-carrier using a standard modulation scheme, for example
PSK, QAM, etc. The sub-carrier frequencies are chosen so that the
modulated data streams are orthogonal to each other, meaning that
cross-talk between the sub-channels is eliminated. Channel equalization
is simplified by using many slowly modulated narrowband signals instead
of one rapidly modulated wideband signal. An advantage of OFDM is its
ability to cope with severe channel conditions, such as multipath and
narrowband interference, without complex equalization filters. As
mentioned, OFDM has developed into a popular scheme for wideband digital
communication systems.

[0046] In OFDM, the sub-carrier frequencies are chosen so that the
modulated data streams are orthogonal to each other, meaning that
cross-talk between the sub-channels is eliminated and inter-carrier guard
bands are not required. This greatly simplifies the design of both the
transmitter and the receiver without a separate filter for each
sub-channel, which is required in conventional FDM. The orthogonality
also allows high spectral efficiency, near the Nyquist rate. The
orthogonality also allows for efficient modulator and demodulator
implementation using the FFT algorithm. Although the principles and some
of the benefits have been known since the 1960s, OFDM is made popular
today for wideband communication by availability of low-cost digital
signal processing components that can efficiently calculate the FFT. OFDM
requires accurate frequency synchronization in the receiver; any
inaccuracy means that the sub-carriers no longer appear orthogonal,
resulting in degraded performance.

[0047] U-TDOA location of devices transmitting these signals becomes a
challenge, as receivers are needed to capture very high bandwidth
signals, store and process them. The requirements for RF signal
bandwidth, digital signal processing power, and memory required to
perform U-TDOA location on a signal with a 20 MHz bandwidth signal can be
six times that required to locate third generation (3G) wireless devices
utilizing signals with a bandwidth of 3 to 5 MHz. These increased
requirements can dramatically increase the cost and complexity of the
Signal Collection System or LMU (the terms SCS and LMU will be used
interchangeably herein).

[0048] With an embodiment of the present invention, TDOA location of a
broadband wireless transmitter is accomplished by selecting only a
portion of the spectrum of the transmitted signal, which can be supported
by the available capability of the SCSs measuring the signal. The
capability includes the level of receiver bandwidth, signal sampling
rate, DSP processing throughput, and memory. As an example, a SCS may be
equipped with an RF receiver containing filters with sufficient bandwidth
to support a 3GPP UMTS waveform (3-5 MHz bandwidth), analog to digital
converters capable of sampling a 3-5 MHz wide signal, and digital signal
processing resources and memory capable of performing TDOA location
processing of a signal with 3-5 MHz of bandwidth, but with the SCS
incapable of collecting and processing a full 20 MHz bandwidth signal. In
this case, a contiguous portion of the transmitted signal may be
selected, with this portion having a signal bandwidth that is within the
capabilities of the SCS. This signal reduction is possible because the
OFDM waveform transmitted by a broadband device actually consists of many
(256 for example) contiguous channels, which can be individually
demodulated and separated from the rest of the signal. A block of 64
channels, which might be selected to be a power of 2 for FFT efficiency,
may be processed in the TDOA location computation. In a direct sequence
spread spectrum system such as IS-95, or UMTS, this would not be
possible, as there would be no way to extract any meaningful data from a
small portion of the transmitted signal. A small portion of the spectrum
could not be demodulated without the rest of the signal as in an OFDM
waveform. Because these are high bandwidth signals, a station-based
process as defined in the '144 patent could be used as this minimizes the
amount of data transferred, although signal data could be transferred to
a central node for central correlation processing, as also described in
the '144 patent. This technique applies to both wideband and narrowband
embodiments of the SCS.

[0049] The transmitted waveform used in the IEEE 802.16 WiMAX system
consists of 256 channels. The outer 55 channels are guard channels in
which minimal energy is transmitted. In addition, there are 8 pilot
channels to aid in signal acquisition and demodulation. Selection of the
bandwidth to process should include a number of pilot channels which are
placed through the full channel set to help the receiver properly detect
and demodulate the signal. In addition, the guard signals are good
candidates to exclude as they contain little useful signal energy. The
channel set selection could also be based upon knowledge of the current
utilization of the spectrum, where less utilized spectrum is chosen for
processing to minimize the likelihood of interference. The selected
channel set may also be chosen to be a power of 2 or 4 to allow for
efficient multiplexing with an FFT.

[0050] A transmitted OFDM waveform is typically constructed as shown in
FIG. 2A. The process may be summarized as follows: [0051] 1. Information
bits are encoded with additional redundant and parity bits to allow the
receiver to detect and correct errors. (Reference numeral 20.) [0052] 2.
Data are interleaved to distribute the redundant bits over a larger time
to allow the redundancy in the error correction codes to correct short
degradations in received signal quality. (Reference numeral 21.) [0053]
3. The encoded bits are modulated into PSK or QAM symbols, in the form of
base-band sample data. (Reference numeral 22.) [0054] 4. A block of PSK
or QAM symbols (256) are passed through an inverse Fast Fourier Transform
(IFFT) creating the OFDM signal. (Reference numeral 23.) [0055] 5. The
digital signal is then converted to analog with a digital to analog
converter. (Reference numeral 24.) [0056] 6. The signal is frequency
converted to Radio Frequency (RF) and then it is transmitted. (Reference
numeral 25.)

[0057] A typical OFDM receiver performs the following steps shown in FIG.
2B. This process is essentially the reverse of the transmitter process:
[0058] 1. RF signal is frequency converted to base-band, filtered, and
digitized. (Reference numerals 26 and 27.) This may include:

[0059] a. One more stages of frequency conversion of the analog signal to
and intermediate frequency (IF), or base-band;

[0060] b. Filtering of the analog signal to a bandwidth which satisfies
the Nyquist criteria for the signal bandwidth, and sample rate;

[0061] c. Digitizing base-band or IF signal with an analog to digital
converter;

[0062] d. Digital down-conversion of IF to base-band if necessary; and

[0068]FIG. 3 shows how only a portion of the transmitted channels of an
OFDM signal is selected for location processing.

[0069]FIG. 4 shows the signal processing chain of the SCS. In an
illustrative example of the present invention, the SCS has RF signals
from antennas connected to the input. These RF signals may contain some
undesired out of band signals from the base station transmitter, or other
interferers. The RF filter 40 reduces the levels of the undesired signals
outside of the pass-band of the desired signals, while allowing the
pass-band signals to pass to the next stage with minimal loss. The
filtered RF signal is then frequency converted 41 to an IF frequency of
around 70 MHz. The frequency conversion process is accomplished by
modulating the RF signal with a sinusoidal Local Oscillator (LO) signal
with a frequency about 70 MHz lower than the desired RF frequency. This
will cause the RF signal to be translated to frequency around 70 Hz.
Adjusting the LO frequency will allow different portions of the LO
frequency to be tuned around 70 MHz. In this case the desired portion of
the receiver RF signal will be tuned to a center frequency of 70 MHz.

[0070] The IF signal is then passed through an IF filter 42 to reduce the
bandwidth of the signal such that it can easily be sampled at a rate
meeting the Nyquist criteria to avoid aliasing. The IF filter 42 has a
pass-band of 5 MHz and a center frequency of 70 MHz. The filter, which
could be made up of one or more cascaded surface acoustic wave (SAW)
filters, reduces the power level of all signals outside of a 10 MHz
bandwidth by 75 dB, relative to the pass-band level. A filter of this
type is selected because many transceivers are designed with a 70 MHz IF
frequency, and filters with a 5 MHz pass-band are commonly used in WCDMA
and cable TV equipment. These filters are inexpensive and readily
available. Passing a wider bandwidth of 20 MHz would likely require a
custom filter design, and increase the SCS cost. The filtered IF signal
is then sampled by the analog to digital converter 43, with a sample rate
of 60 MSPS. A high sampling rate permits the use of digital down
converters with output signal sample rates of 12 MSPS. The digitized
signal is then passed through a digital down converter 44, where the
digital signal is filtered to a bandwidth of <5 MHz, and converted
from IF to base-band. In this process the sample rate is also decimated
to 12 MSPS. The decimation eliminates the redundant samples, reducing the
processing load on the DSP 45.

[0071] The largest savings from reducing the processes spectrum is in
memory and DSP processing throughput. The required memory and DSP
throughput can be compared when performing a TDOA measurement on the full
20 MHz signal, which would have a sample rate of 48 MHz vs. a 5 MHz
portion of the signal, with a sample rate of 12 MHz. TDOA measurements
are made by performing a cross correlation of the signal received by one
SCS with a reference signal received at another SCS, as a function of
time difference, as shown below.

y(r)=Σnx(τ)r(n+τ)

[0072] where x(n) is the received signal, r(n) is the reference signal,
and N is the number of samples in both the received and reference
signals.

[0073] Both the size of memory required to store the signal, as well as
the number of multiplications to perform the correlation are a linear
function of the number of samples. If one second of received and
reference data are to be used for correlation, the full signal would
require the storage of 48 million samples, and 48 million multiplications
to compute a single cross-correlation value. The reduced signal would
require storage of 12 million samples, and 12 million multiplications to
compute a single correlation value. The reduced signal requires only 1/4
the memory and DSP 45 power as the full signal.

[0074]FIG. 5 shows the demodulation and decoding by the primary SCS in a
station-based processing implementation. Because much of the underlying
data is missing due to the reduction in processed spectrum, the steps of
interleaving and decoding are not feasible, and are eliminated, further
reducing required processing. [0075] 1. RF signal is frequency converted
to base-band, filtered, and digitized. (Reference numerals 50 and 51.)
This may include:

[0076] a. One more stages of frequency conversion of the analog signal to
and intermediate frequency (IF), or baseband;

[0077] b. Filtering of the analog signal to a bandwidth which satisfies
the Nyquist criteria for the signal bandwidth, and sample rate, [0078]
i. The sample rate is be much lower than the 48 MHz required to properly
sample a 20 MHz signal; [0079] ii. The filter bandwidth could be much
less than the 20 MHz required to pass an entire signal;

[0080] c. Digitizing base-band or IF signal with an analog to digital
converter;

[0085] The reconstruction process used for the reduced signal is shown in
FIG. 6. [0086] 1. The encoded bits are modulated into PSK or QAM symbols,
in the form of base-band sample data. (Reference numeral 60.) [0087] 2. A
block of PSK or QAM symbols (256) are passed through an inverse Fast
Fourier Transform (IFFT) creating the OFDM signal. (Reference numeral
61.) [0088] 3. Additional characteristics are applied to the signal, such
as phase corrections. (Not shown.)

[0089] Therefore, the station-based TDOA location process for the reduced
waveform would be as shown in FIG. 7: [0090] 1. The primary SCS, as well
as cooperating SCSs receive and digitize the transmitted signal
(reference numeral 70):

[0091] a. Sampling of the receive signals is synchronized to facilitate
TDOA processing.

[0092] b. Sampled signal bandwidth and sample rate may be reduced, as only
a fraction of the signal bandwidth will be processed. [0093] 2. The
primary SCS implements the demodulation steps above, which excludes
de-interleaving and error correction decoding, and also measures other
signal characteristics, such as phase corrections. (Reference numeral
71.) [0094] 3. Encoded bits and characteristic data is transferred to
cooperating SCSs. (Reference numeral 72.) [0095] 4. Primary and
cooperating SCSs reconstruct the reference base-band signal, by
implementing the steps shown in FIG. 6. (Reference numeral 73.) [0096] 5.
Primary and cooperating SCSs perform correlation processing to measure
the Time Difference of Arrival of the signal, and send the TDOA
measurement to the TLP. (Reference numeral 74.) [0097] 6. TLP computes
the location. (Reference numeral 75.)

[0098] The concepts described herein are not limited to WiFi or WiMAX
systems, but apply to any system which uses OFDM for communication. The
invention is not limited to the specific architecture and/or
implementation defined for the SCS.

[0099] Alternate Embodiments

[0100] An extension to the above approach allows the use of a
narrower-band front end to capture just a portion of the OFDM waveform
spectrum as described above, while maintaining the improved multi-path
resolution that can be achieved using the wider-band waveform that is
transmitted by the mobile device. This extension involves sampling a
portion of the OFDM waveform spectrum as described above for a interval
of time, then re-tuning the frequency agile receiver to sample a
different portion of the OFDM waveform spectrum in the next interval of
time, then continuing this process to get multiple slices of the OFDM
waveform spectrum (up to covering the entire OFDM waveform spectrum with
a series of narrow-band samples). This re-tuning can be performed in
sequential or random patterns to cover most or all of the OFDM waveform
bandwidth. This is illustrated in FIG. 9. (See also, U.S. Pat. No.
6,646,604, Nov. 11, 2003, "Automatic Synchronous Tuning of Narrowband
Receivers of a Wireless Location System for Voice/Traffic Channel
Tracking," which is hereby incorporated by reference in its entirety.)

[0101] The ability to resolve multi-path components in the
cross-correlation function used to measure TDOA values is limited by the
bandwidth of the signal that is used. When there is a direct path signal
and a delayed signal that arrives in close proximity in time, an ideal
correlation function using infinite bandwidth signals would result in two
peaks that are easily resolvable as shown in FIG. 8A. When band limited
signals are used to generate the cross-correlation function, these peaks
are "smeared" by a smoothing function whose width is proportional to the
inverse of the bandwidth of the signal. When this inverse bandwidth is
wider then the separation between the arriving signals, they become
indistinguishable as shown in FIG. 8B. If, however, this inverse
bandwidth is narrower then the separation between the arriving signals,
then the peaks in the correlation function, while still smeared, can be
easily distinguished, as shown in FIG. 8c, where the bandwidth is 4 times
that of the signal in FIG. 8B. The ability to distinguish the different
signal arrivals allows the selection of the direct path signal. This
provides a more accurate TDOA measurement, directly reducing error of the
location estimate.

[0102] This advantage of a wider bandwidth cross-correlation function can
be achieved without the added cost of sampling the full bandwidth
simultaneously, which would require a wider-band front receiver, higher
sample rate A/D converter, more storage, and processing power. Instead, a
series of narrow-band samples can be stored, and the advantage of the
wider bandwidth cross-correlation function can be achieved using the
bandwidth synthesis process described in U.S. Pat. No. 6,091,362, Jul.
18, 2000, "Bandwidth Synthesis for Wireless Location System," which is
hereby incorporated by reference in its entirety.

[0103] In frequency hopped waveforms such as GSM, the advantage gained by
performing bandwidth synthesis can be somewhat limited by the fact that
the spacing of the sampled frequency is not contiguous in general, and
can be quite sparse in practice. This sparse spacing results in
ambiguities in the synthesized cross-correlation function that may not be
successfully resolved. In this embodiment, the OFDM waveform occupies a
large contiguous block of spectrum which is sampled using a series of
narrower slices of the spectrum. This insures that the slices will be
adjacent to each other in frequency (see FIG. 9), allowing the bandwidth
synthesis process to produce a synthesized cross-correlation function
that does not contain ambiguities.

[0104] C. Conclusion

[0105] The true scope the present invention is not limited to the
presently preferred embodiments disclosed herein. For example, the
foregoing disclosure of a presently preferred embodiment of a Wireless
Location System uses explanatory terms, such as Signal Collection System
(SCS), TDOA Location Processor (TLP), Applications Processor (AP),
Location Measuring Unit (LMU), and the like, which should not be
construed so as to limit the scope of protection of the following claims,
or to otherwise imply that the inventive aspects of the Wireless Location
System are limited to the particular methods and apparatus disclosed.
Moreover, as will be understood by those skilled in the art, many of the
inventive aspects disclosed herein may be applied in location systems
that are not based on TDOA techniques. For example, the invention is not
limited to systems employing SCS's constructed as described above. The
SCS's, TLP's, etc. are, in essence, programmable data collection and
processing devices that could take a variety of forms without departing
from the inventive concepts disclosed herein. Given the rapidly declining
cost of digital signal processing and other processing functions, it is
easily possible, for example, to transfer the processing for a particular
function from one of the functional elements (such as the TLP) described
herein to another functional element (such as the SCS) without changing
the inventive operation of the system. In many cases, the place of
implementation (i.e., the functional element) described herein is merely
a designer's preference and not a hard requirement. Accordingly, except
as they may be expressly so limited, the scope of protection of the
following claims is not intended to be limited to the specific
embodiments described above.

Patent applications by Rashidus S. Mia, Phoenixville, PA US

Patent applications by Robert J. Anderson, Phoenixville, PA US

Patent applications by TRUEPOSITION, INC.

Patent applications in class Plural channels for transmission of a single pulse train

Patent applications in all subclasses Plural channels for transmission of a single pulse train